How do I handle my best friend's boyfriend being disrespectful to me when they are fighting?

There is nothing you can say or do that will change this guy or his opinion. The ball is in your friend's court. When she gets sick of this insecure little boy things will hopefully change and you won't need to ask this question. But expect him to make it difficult for your friend to continue being friends with you until he's gone. He's the type that doesn't share well, if you get my meaning.